The technique of digitally encoding a video signal and particularly a composite video signal, is well known. Sampling pulses are generated in synchronism with a color video burst signal. The sampling pulses have a repetition rate that is a multiple of the burst signal frequency. Each sample of the color video signal is encoded, or digitized, such as by pulse code modulation (PCM). Digitally encoded video signals are used in time base error correction devices, noise suppression devices, the addition of various special video effects and the like. Digitally encoded video signals are particularly advantageous for recording and reproduction and also for special types of transmission.
In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. Since the phase of the color subcarrier signal is used to represent color information, it is important that, when digitally encoding the color video signal, the phase of the sampling pulses be accurately controlled. Undesired phase shifts, such as may be due to temperature drift, aging of the electrical components, and the like, may result in a phase error in the sampling pulse relative to the chrominance subcarrier signal which has the effect of distorting or interfering with the overall chrominance effect of the video picture which ultimately is reproduced from the digitally encoded video signal.
To identify the aforementioned phase shifts between the sampling pulses and the chrominance subcarrier signal, the instantaneous phase angle of the burst signal at the time of sampling is determined. If the phase angle of the burst signal differs from a desired phase angle, the phase of the sampling pulses may be adjusted accordingly. If the phase angle of the burst signal is different than an expected phase angle, the phase difference between the modulated chrominance information and the signal will also be in error, thereby causing distortion of the color within an output video signal.
It is therefore important to the operation of the video transmission system that the burst signal is separated correctly from the composite video signal in order that the phase of the burst signal can be compared to the phase of a reference signal. Any error in separating the burst signal from the input composite video signal may result in an error determining the phase difference between the two signals and will cause the output video signal to be in error.
A composite color video signal includes horizontal synchronizing signals, a burst signal superimposed onto the pedestal level at the back porch of the horizontal synchronizing signal and a video information signal. The video information signal comprises a chrominance subcarrier having different phases amplitude-modulated with chrominance information. The composite color video signal includes both luminance and chrominance information. Separator circuits are utilized to separate the horizontal synchronizing signal and the burst signal from the incoming video signal. The burst signal has a burst signal frequency equal to 3.58 MHz, which is the frequency of the chrominance subcarrier f.sub.SC.
Within the composite video signal the values of the components are determined by their relative amplitude with respect to the blank or pedestal level. It is therefore essential that the blank or pedestal level is maintained at a known level so that the value of the component of the composite video signal can be readily determined. In the past, the blank level has been set to a known DC level allowing the values of the components of the composite video signal to be determined by determining their amplitude with respect to the blank level. However, care must be taken when setting the blank level to a specific value that the remainder of the composite video signal is not altered. Video systems of the prior art, in order to set the blank level to a known value, would separate the chrominance and luminance information from the composite video signal and then hard-clamp the blank level of the composite video signal to the appropriate level. Such a system is disadvantageous because at least two additional pins on the integrated circuit and external components are required within the system.
The video signal can also be separated into a separate chrominance or C signal and separate luminance or Y signal. A properly configured television, monitor or display will accept the separate chrominance C and luminance Y signal.
The composite video signal contains information which is used by a video system to generate a video picture on a display, monitor or television. Each period, within the horizontal portion of a composite video signal contains information representing one horizontal output line which is to be output on the video display, monitor or television. Each horizontal period includes a horizontal synchronization pulse, a burst signal and a video information signal. In many video transmission systems, color or chrominance information is represented by a particular phase of the chrominance subcarrier signal that is amplitude modulated with color information. The horizontal synchronization pulse is used by a phase-locked loop to synchronize the system for displaying the next horizontal line of video information. The burst signal is used to synchronize the phase of the sampling pulses with the phase of the color subcarrier signal.
A video picture or frame is made up of a number of horizontal lines included within the video display. To display a video picture or frame the video system begins at the top of the screen and displays the information within the composite video signal one horizontal line at a time. The information for each horizontal line is contained within a horizontal period of the composite video signal. After each horizontal period, the video system moves to the next line and displays the information within the next horizontal period of the composite video system. This continues until the video system reaches the bottom line on the video display. After displaying the video information on the bottom line of the video display, the video system must reset itself to the top of the display in order to begin displaying the next frame. In order to allow the system to reset itself to the top of the video display a vertical blanking period is included within the composite video signal, after the video information for each frame. This vertical blanking period allows the video system to reset to the top of the video display and begin displaying the information for the horizontal lines of the next frame. Therefore, a number of horizontal periods, enough to comprise a frame or screen, are strung together, within the composite video signal. Between each frame, the composite video signal includes a vertical blanking period which allows the video system to perform a vertical reset and prepare to display the next frame by moving back up to the top of the video display.
During the vertical blanking period the composite video signal includes a first period of equalizing pulses, a period of serration pulses and a second period of equalizing pulses. During this vertical blanking period the video system resets itself to the top of the video display so that it is ready to begin displaying the information for the next frame. However, the video system must be notified of or be able to detect the vertical blanking period so that it can reset itself to the top of the video display. The serration pulses carry synchronization information used by the local vertical oscillator, within the video system, during a vertical reset.
The equalizing and serration pulses during the vertical blanking period are all generated at a frequency equal to twice the frequency of the horizontal synchronizing pulses. A sync separator circuit is used to separate all of the synchronization pulses from the composite video signal including the horizontal, equalizing and serration pulses. However, the sync separator circuit separates the synchronization pulses by comparing their amplitude with respect to the blank level of the signal and therefore has no way of differentiating between horizontal synchronization pulses, equalizing pulses and serration pulses. The output of the sync separator circuit is used by the horizontal phase-locked loop to lock the video system in phase with the composite video signal during the horizontal period of each frame. During the vertical blanking period, the sync separator circuit will output the equalizing and serration pulses which are generated at twice the frequency of the horizontal synchronization pulses. Thus, twice as many synchronization pulses are generated during the vertical blanking period as during the horizontal period.
A mixer circuit, as illustrated in FIG. 1, mixes two input signals together in a predetermined ratio forming an output signal. The mixing of the two input signals is controlled by a control signal which specifies the ratio of the input signals. Input signals Input1 and Input2 are coupled as inputs to the mixer circuit 10. A control signal Control is coupled as a control input to the mixer circuit 10 for controlling the mixing ratio of the two input signals Input1 and Input2. The mixer circuit 10 outputs an output signal Output which is a combination of the two input signals Input1 and Input2 in a predetermined ratio, as specified by the control signal Control. In this manner the mixer circuit 10 combines the two input signals Input1 and Input2 into a single output signal Output.
When either or both of the two input signals Input1 and Input2 are provided to the mixer circuit 10 from a separate integrated circuit, noise may be introduced into the output signal Output causing the output signal Output to be in error or perhaps distorted. This noise results because the level of the input signals Input1 and Input2 will each be in reference to a different ground reference signal when coupled from separate integrated circuits. The mixer circuit 10 has a separate ground reference signal. Each of the ground reference signals corresponding to each of the input signals Input1 and Input2 may be different than the ground reference signal of the mixer circuit 10. Thus, the level of each input signal Input1 and Input2 may be correct with respect to its own ground reference signal but may be in error with respect to the ground reference signal of the mixer circuit 10. Accordingly, when the input signals Input1 and Input2 are combined by the mixer circuit 10 generating the output signal Output, the different ground reference levels of each of the signals, will cause noise or distortion to be introduced into the output signal Output. This noise or distortion will negatively impact the quality of the output signal Output.